There's kind of a fun brain teaser related to this. Imagine you're speaking to an entirely alien species in another dimension. And you're trying to describe "left" to them. It's impossible.
I say it's a brain teaser only because this sounds highly improbable at first. Forwards and backwards can be described, and even up/down if we assume basic fundamental rules remain the same. Yet left and right? It seems impossible for it to be impossible.
It turns out you can define it from looking at radioactive decay (or any other interaction governed by the weak force). Feynman famously had a short description that ended with the punch line
> Suppose, after lots of communication you finally can go into space and meet your alien counterpart. If, as you approach one another, the alien extends its left hand to shake, watch out! He’s made of antimatter!
because even the physics of weak decays of will invert in an antimatter universe.
I think it should also be possible with electro magnetism. By first defining the sign convention for electron flow and then deriving left hand and right hand rules of magnetism or something like that. It’s a bit tricky to explain the sign convention, but I think if one sticks with the movement of elementary particles, it should work.
Which claim? The full transcript of the Feynman story is available online [1], you can search for "martian" to get right to the story. It's presented in the context of explaining CP symmetry, which says that physics is conserved under a parity (mirror reflection) plus charge inversion (matter -> antimatter) transformation. Of course it's only approximate [2].
I got the quote slightly differently from what is recorded on the caltech sight, the one they have is
> So if our Martian is made of antimatter and we give him instructions to make this “right” handed model like us, it will, of course, come out the other way around. What would happen when, after much conversation back and forth, we each have taught the other to make space ships and we meet halfway in empty space? We have instructed each other on our traditions, and so forth, and the two of us come rushing out to shake hands. Well, if he puts out his left hand, watch out!
So the claim is that CP violation would make it possible to define "left" without ever pointing at things? Of course then the question would be what this definition actually is.
The assertion that it is possible or the attribution to Feynman? Scientific American [1] references Feynman's 4th Messenger lecture at Cornell: "Symmetry in Physical Law" (1964) [2] [3]
It isn't impossible, it is just arbitrary. The whole reason we can have this huge thread about different names for directions is that they is just arbitrary labels, with groups of people having a consensus on what word to use.
BTW, If you really must explain it without just saying it is arbitrary, you can talk about clockwise vs. counterclockwise... which came from sundials, which came from the direction our shadows move as the sun moves. In a similar vein, Left goes against the shadows, Right follows the shadows.
Shadows come from the rotation of the earth, not its orbit. But you are still correct - if the earth suddenly started having inconsistency in the direction it spins, that explanation would fall apart.
I think this is due to the bilateral symmetry of most animals. But imagine being non-symmetric yourself (or the alien race being non-symmetric). The concepts of left and right seem to become much more communicable now.
Yeah the handedness of a 3D coordinate system is a pretty arbitrary low level choice. You don’t even need to talk to aliens, just try implementing some geometric algorithm in Unity which uses a left-handed system.
One approach would be: you're moving along a straight line, perpendicular to a large body that attracts you through gravity.
Now "down" is towards the gravity, "up" is the opposite direction, and "left" and "right" are perpendicular, but which one is left and right, if you cannot rely on handedness?
That helps if you meet them in person, but if you communicate through radio waves, they'll have no way to know how to decode signal into an image with the intended left/right and up/down orientation
I say it's a brain teaser only because this sounds highly improbable at first. Forwards and backwards can be described, and even up/down if we assume basic fundamental rules remain the same. Yet left and right? It seems impossible for it to be impossible.